High temperature thermochemical energy storage system

ABSTRACT

A thermochemical energy storage system and method of storing thermal energy are described. The energy storing system described herein comprises a reactor comprising: a) a reactor with a CO2 sorbent including MgO; and b) a supercritical CO2 source with supercritical CO2 and H2O, wherein the supercritical CO2 source is in fluid communication with the reactor and the CO2 sorbent including MgO to allow flow of the supercritical CO2 and H2O between the supercritical CO2 source and the reactor, thereby allowing contact of CO2 with the CO2 sorbent comprising MgO.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/734,432, filed Sep. 21, 2018, which is hereby incorporated byreference in its entirety.

FEDERALLY SPONSORED RESEARCH

This invention was made with government support under DE-EE0008126awarded by Office of Energy Efficiency & Renewable Energy of the U.S.Department of Energy. The government has certain rights in theinvention.

BACKGROUND

Societal energy demands are constantly increasing while fossil fuelresources, the main energy resource of many national energy systems, arelimited and predicted to become scarcer, and as a result to become moreexpensive in coming years. Furthermore, many concerns exist regardingthe environmental impacts associated with continuous drilling andpumping of the fossil fuels from the Earth's crust and increasing energyconsumption. Specifically, concerns have been raised regarding thepossible effect of increased use of fossil fuels on climate change andatmospheric pollution.

Changes are required in energy systems, partly through the adoption ofadvanced energy technologies and systems to address these seriousenvironmental concerns. The anticipated worldwide increase in energydemand and concern regarding environmental problems has become a drivingforce for the utilization of more efficient and cleaner energytechnologies. Examples include advanced systems for waste energyrecovery and energy integration. Important technologies that cancontribute to avoiding environmental problems and increasing theefficiency of energy consumption include thermal energy storage (TES),and more specifically, thermochemical energy storage (TCES).

Thermal energy storage is especially an important technology in systemsinvolving renewable energy sources as well as other energy resources asit can make their operation more efficient. One example is bridging theperiods between when energy is harvested and when it is needed. Forexample, the next generation of advanced concentrating solar power (CSP)plants are being designed to increase the sunlight to electricityconversion efficiency, and one of the major techniques to enact thisincrease is through the use of receivers, heat transfer fluids (HTF),thermal energy storage systems, and power blocks that operate at hightemperatures. It was found that CSP systems, for example, requirethermal energy storage to be competitive with conventional grid scalepower generation systems. Thus, TES can play an important role inincreasing the contribution of various types of renewable energy in theenergy production of regions and countries.

Various TES technologies and applications exist. The selection of a TESsystem for a particular application depends on many factors, includingstorage duration, economics, supply and utilization temperaturerequirements, storage capacity, heat loss and available space.

More compact TES can be achieved based on a system that utilize chemicalreactions. However, the current-state-of the-art molten salt basedthermal storage systems are unable to operate in the high temperaturerange required, for example, in CSP systems. High temperature thermalenergy is generally stored as sensible heat in either molten salt orsynthetic organic heat transfer oil. However, these mediums store heatin a very low volumetric energy density and are not able to store heatabove 500° C.

Therefore, thermochemical energy storage systems exhibiting very highvolumetric energy density and capable of operating through a widetemperature range are needed. Even further, improved methods for storingenergy would be desirable.

Accordingly, such thermochemical energy storage systems and methods forstoring energy are described herein.

SUMMARY OF THE INVENTION

Disclosed herein is a system for storing energy comprising: a) a reactorcomprising a CO₂ sorbent comprising MgO; and b) a supercritical CO₂source comprising supercritical CO₂ and H₂O, wherein the supercriticalCO₂ source is in fluid communication with the reactor and the CO₂sorbent comprising MgO to allow flow of the supercritical CO₂ and H₂Obetween the supercritical CO₂ source and the reactor, thereby allowingcontact of CO₂ with the CO₂ sorbent comprising MgO.

Also disclosed herein disclosed herein is a system for storing energycomprising: a) a reactor comprising a CO₂ sorbent comprising MgO and aliquid carbonate promoter; and b) a supercritical CO₂ source comprisingsupercritical CO₂, wherein the supercritical CO₂ source is in fluidcommunication with the reactor and the CO₂ sorbent comprising MgO toallow flow of the supercritical CO₂ between the supercritical CO₂ sourceand the reactor, thereby allowing contact of CO₂ with the CO₂ sorbentcomprising MgO.

Also disclosed herein is a system for storing energy comprising: a) areactor comprising a CO₂ sorbent comprising MgO and a liquid carbonatepromoter; and b) a supercritical CO₂ source comprising supercritical CO₂and H₂O, wherein the supercritical CO₂ source is in fluid communicationwith the reactor and the CO₂ sorbent comprising MgO to allow flow of thesupercritical CO₂ and H₂O between the supercritical CO₂ source and thereactor, thereby allowing contact of CO₂ with the CO₂ sorbent comprisingMgO.

Also disclosed herein is a method for storing energy comprising thesteps of: a) in a reactor, in the presence of H₂O and/or a carbonate,heating MgCO₃ with supercritical CO₂ having a temperature of at least450° C., thereby promoting an endothermic chemical reaction to produceCO₂ and MgO; and b) separating the CO₂ from the MgO.

Also disclosed herein is a method of storing energy comprisingcontacting MgCO₃ with supercritical CO₂ having a temperature of at least450° C. in the presence of H₂O and/or a carbonate in the systemdisclosed herein, to thereby store energy.

Additional advantages will be set forth in part in the description whichfollows, and in part will be obvious from the description, or can belearned by practice of the aspects described below. The advantagesdescribed below will be realized and attained by means of the chemicalcompositions, methods, and combinations thereof particularly pointed outin the appended claims. It is to be understood that both the foregoinggeneral description and the following detailed description are exemplaryand explanatory only and are not restrictive of the invention, asclaimed.

DESCRIPTION OF THE FIGURES

The accompanying figures, which are incorporated in and constitute apart of this specification, illustrate several aspects, and togetherwith the description, serve to explain the principles of the invention.

FIG. 1 shows a process flow diagram of a TCES system described herein.

FIG. 2 show an exemplary design of a reactor disclosed herein, such as aheat exchange reactor.

FIG. 3 shows an isothermal cycle of a method disclosed herein.

FIG. 4 shows a gas-solid reaction equilibrium of MgO/MgCO₃.

FIG. 5 shows the energy of a system disclosed herein with allirreversibles or reactor irreversibles only.

FIG. 6 shows a plot of the equilibrium CO₂ partial pressure vs.temperature for MgCO₃ within a supercritical CO₂ power cycle.

FIG. 7 shows the itemized cost versus energy storage density of thematerial. Assuming a d=16 [m] shaft diameter.

FIG. 8 shows the behavior of a MgO sorbent over 150 cycles.

FIG. 9 shows the a non-limiting example of the process of making MgOsorbents disclosed herein.

FIG. 10 shows a non-limiting example of a reactor that was used inexperiments.

FIG. 11 shows a run chart for four cycles at 4 hr cycle times at anequilibrium temperature around 650° C.

DETAILED DESCRIPTION

The present invention can be understood more readily by reference to thefollowing detailed description of the invention.

Disclosed herein are systems, materials, compounds, compositions, andcomponents that can be used for, can be used in conjunction with, can beused in preparation for, or are products of the disclosed method andcompositions. It is to be understood that when combinations, subsets,interactions, groups, etc. of these materials are disclosed that whilespecific reference of each various individual and collectivecombinations and permutation of these compounds cannot be explicitlydisclosed, each is specifically contemplated and described herein. Thisconcept applies to all aspects of this disclosure including, but notlimited to, steps in methods of making and using the disclosedcompositions. Thus, if there are a variety of additional steps that canbe performed it is understood that each of these additional steps can beperformed with any specific aspect or combination of aspects of thedisclosed methods, and that each such combination is specificallycontemplated and should be considered disclosed.

All publications mentioned herein are incorporated herein by referenceto disclose and describe the methods and/or materials in connection withwhich the publications are cited.

1. Definitions

In this specification and in the claims that follow, reference will bemade to a number of terms that shall be defined to have the followingmeanings:

Throughout this specification, unless the context requires otherwise,the word “comprise,” or variations such as “comprises” or “comprising,”will be understood to imply the inclusion of a stated integer or step orgroup of integers or steps but not the exclusion of any other integer orstep or group of integers or steps.

It must be noted that, as used in the specification and the appendedclaims, the singular forms “a,” “an” and “the” include plural referentsunless the context clearly dictates otherwise. Thus, for example,reference to “a system” includes combination of two or more suchsystems, and the like.

“Optional” or “optionally” means that the subsequently described eventor circumstance can or cannot occur, and that the description includesinstances where the event or circumstance occurs and instances where itdoes not.

“Consists essentially of” limits the scope to the specified materials(i.e. MgO) or steps and those that do not materially affect the basicand novel characteristic(s) of the claimed invention.

“Consisting of” limits the scope to the specified materials (i.e. MgO).

Ranges can be expressed herein as from “about” one particular value,and/or to “about” another particular value. When such a range isexpressed, another aspect includes from the one particular value and/orto the other particular value. Similarly, when values are expressed asapproximations, by use of the antecedent “about,” it will be understoodthat the particular value forms another aspect. It will be furtherunderstood that the endpoints of each of the ranges are significant bothin relation to the other endpoint, and independently of the otherendpoint.

The term “thermal energy storage” as defined herein is referenced to asystem capable of temporary holding thermal energy in substances forlater utilization.

The term “sensible thermal energy storage” as defined herein isreferenced to energy stored in vibrational modes of molecules. SensibleTES systems store energy by changing the temperature of the storagemedium, which can be water, brine, rock, soil, concrete, sand, moltensalt and the like.

The term “latent thermal energy storage” as defined herein is referencedto energy stored in medium as it changes phase, for example cold storagewater/ice and heat storage by melting paraffin waxes.

The term “thermochemical energy storage” is referred to energy stored inchemical bonds of molecules, or in the reaction between the reactants.For example, metal oxides, reversible reduction oxidation reactions, andthe like. Thermochemical energy storage can also include a system thatallows separation of reactants that can be subsequently combined againin exothermic reaction. For example, the separation and laterre-combination of CO₂ and MgO.

The term “sorbent” as used herein is referred to a solid materialcapable of absorbing \ liquids or gases via fluid-solid chemicalreaction

The term “heat exchange reactor” as used herein is referred to a reactorused to transfer heat between one or more fluids. The fluids can beseparated by a solid wall to prevent mixing or they can be in directcontact.

The term “adiabatic reactor” as used herein is referred to a reactorthat utilizes an adiabatic process that occurs without loss of heat, ormatter, between the reactor and its surroundings.

The terms “gas expander” or “turbo expander,” or “expansion turbine” or“turbine” can be used interchangeably and are referred to a centrifugalor axial flow turbine through which a high pressure gas is expanded toproduce work.

The term “exothermic reaction” as referred herein is a chemical reactionthat releases energy by heat.

The term “endothermic reaction” as referred herein is a reaction inwhich the system absorbs energy from its surroundings. In some aspects,the absorbed energy is in the form of heat.

A supercritical fluid as described herein is a fluid at a temperatureabove its critical temperature and at a pressure above its criticalpressure. A supercritical fluid exists at or above its “critical point,”the point of highest temperature and pressure at which the liquid andvapor (gas) phases can exist in equilibrium with one another. Abovecritical pressure and critical temperature, the distinction betweenliquid and gas phases disappears. A supercritical fluid possessesapproximately the penetration properties of a gas simultaneously withthe solvent properties of a liquid. Accordingly, supercritical fluidextraction has the benefit of high penetrability and good solvation.

As used herein, a fluid which is “supercritical” (e.g. supercriticalCO₂) indicates a fluid which would be supercritical if present in pureform under a given set of temperature and pressure conditions.

A weight percent of a component, unless specifically stated to thecontrary, is based on the total weight of the formulation or compositionin which the component is included.

Disclosed are compounds, compositions, and components that can be usedfor, can be used in conjunction with, can be used in preparation for, orare products of the disclosed methods and compositions. These and othermaterials are disclosed herein, and it is understood that whencombinations, subsets, interactions, groups, etc. of these materials aredisclosed that while specific reference of each various individual andcollective combinations and permutation of these compounds may not beexplicitly disclosed, each is specifically contemplated and describedherein. For example, if a number of different polymers and agents aredisclosed and discussed, each and every combination and permutation ofthe polymer and agent are specifically contemplated unless specificallyindicated to the contrary. Thus, if a class of molecules A, B, and C aredisclosed as well as a class of molecules D, E, and F and an example ofa combination molecule, A-D is disclosed, then even if each is notindividually recited, each is individually and collectivelycontemplated. Thus, in this example, each of the combinations A-E, A-F,B-D, B-E, B-F, C-D, C-E, and C-F are specifically contemplated andshould be considered disclosed from disclosure of A, B, and C; D, E, andF; and the example combination A-D. Likewise, any subset or combinationof these is also specifically contemplated and disclosed. Thus, forexample, the sub-group of A-E, B-F, and C-E are specificallycontemplated and should be considered disclosed from disclosure of A, B,and C; D, E, and F; and the example combination A-D. This conceptapplies to all aspects of this disclosure including, but not limited to,steps in methods of making and using the disclosed compositions. Thus,if there are a variety of additional steps that can be performed it isunderstood that each of these additional steps can be performed with anyspecific embodiment or combination of embodiments of the disclosedmethods, and that each such combination is specifically contemplated andshould be considered disclosed.

2. System

Disclosed herein is a system that can allow affordable and efficientstorage of energy. It is desirable to obtain a thermal energy storagesystem having a high cyclic durability, high volumetric energy density,capable of operating throughout a wide temperature range, while stillproviding economic feasibility. An advantage of thermochemical energystorage systems over other energy storage systems is the smalltemperature range (ΔT) over which the charge and discharge cycle occurs,typically around 25-50° C. This small temperature range allows for highexergetic round-trip efficiency, and has the potential to couple wellwith power cycles that approximate the Carnot ideal of heat transfer atconstant temperature. The system disclosed herein utilizes a reversiblegas-solid reaction, wherein the claimed reaction has substantially nopotential side reaction, and the reactants and/or products are non-toxicand non-corrosive. The energy storage described herein uses highlyreversible and highly energetic gas-solid reaction to store energy on athermochemical basis.

Accordingly, disclosed herein is a system for storing energy comprising:a) a reactor comprising a CO₂ sorbent comprising MgO; and b) asupercritical CO₂ source comprising supercritical CO₂ and H₂O, whereinthe supercritical CO₂ source is in fluid communication with the reactorand the CO₂ sorbent comprising MgO to allow flow of the supercriticalCO₂ and H₂O between the supercritical CO₂ source and the reactor,thereby allowing contact of CO₂ with the CO₂ sorbent comprising MgO.

Also disclosed herein disclosed herein is a system for storing energycomprising: a) a reactor comprising a CO₂ sorbent comprising MgO and aliquid carbonate promoter; and b) a supercritical CO₂ source comprisingsupercritical CO₂, wherein the supercritical CO₂ source is in fluidcommunication with the reactor and the CO₂ sorbent comprising MgO toallow flow of the supercritical CO₂ between the supercritical CO₂ sourceand the reactor, thereby allowing contact of CO₂ with the CO₂ sorbentcomprising MgO. In one aspect, the system for storing energy comprises:a) a reactor comprising a CO₂ sorbent comprising MgO and a liquidcarbonate promoter; and b) a supercritical CO₂ source comprisingsupercritical CO₂ and H₂O, wherein the supercritical CO₂ source is influid communication with the reactor and the CO₂ sorbent comprising MgOto allow flow of the supercritical CO₂ and H₂O between the supercriticalCO₂ source and the reactor, thereby allowing contact of CO₂ with the CO₂sorbent comprising MgO.

The systems disclosed herein utilize a highly reversible gas-solidcarbonation-decarbonation reaction cycle using a CO₂ sorbent. In someaspects, a high temperature carbonation-decarbonation cycle can be basedon the reaction shown in Scheme 1:

Cycle 1: MgO+CO₂↔MgCO₃+108 KJ  (Scheme 1)

It is understood that the described cycle is based on the reversiblereactions of carbon dioxide with solid magnesium oxide. The magnesiumoxide carbonates to form solid magnesium carbonate. In certain aspects,the sorbents described herein can undergo, without degradation, repeatedendothermic-exothermic carbonation cycles at a described abovetemperature range in a closed loop system.

The gas-solid reaction equilibrium of MgO/MgCO₃ is shown in FIG. 4.

The systems disclosed herein include one or more promoters. In oneaspect, the one or more promoters can be one or more kinetic promoters.In another aspect, the one or more promoters can be one or morethermodynamics promoters. The one or more promotors can be a liquid orgaseous. In one aspect, the one or more promoters can bethermodynamically stable.

In one aspect, H₂O is present in the system and acts a promoter. Asdescribed herein, the H₂O can be present together with supercritical CO₂in the supercritical CO₂ source. The H₂O is added as a liquid to thesupercritical CO₂ source. The addition of the H₂O provides severaladvantages in the system. One issue that can reduce the efficiency ofthe system is sintering of MgO/MgCO₃ during the decarbonation phase. Theuse of H₂O as a promoter useful, because increasing P_(H2O) increasesreaction kinetics during carbonation (see Scheme 1). Then the H₂Ominimizes the sintering of MgO/MgCO₃ P_(H2O) is decreased duringdecarbonation phase (see Scheme 1). A non-limiting example of the use ofH₂O as a promoter is as follows: 1) H₂O is added to the supercriticalCO₂ source, which then comprises supercritical CO₂ and H₂O; 2) thetemperature of the supercritical CO₂ source, which then comprisessupercritical CO₂ and H₂O is increased in temperature to expand the CO₂and along with the H₂O is transferred to the reactor; 3) after dischargeof the system, the H₂O is stagnant in the reactor helping to promote thekinetics of the carbonation in the 2^(nd) reaction regime, which isconsidered slow; 4) during decarbonation (system charging) evolved CO₂carries the H₂O away with it back to the supercritical CO₂ source whereit condenses; 5) when the reactor is sitting ideal in the carbonatedstate (charged) P_(H2O) and sintering rate is at a minimum. Thus, theuse of H₂O as a promoter provides for a functionality that allows for anincrease in kinetics during carbonation, but avoids excessive sinteringduring decarbonation.

In one aspect, as described herein, a carbonate is present in thereactor and acts as a promoter. The carbonate is in liquid form. Acarbonate promoter promote the kinetics of the carbonation in the 2^(nd)reaction regime, which is considered slow. Carbonate promoters providefor increased kinetics in the reactor and are thermodynamically stable.In one aspect, the carbonate is selected from the group consisting ofsodium carbonate, lithium carbonate, and potassium carbonate, or amixture thereof. For example, the carbonate can be sodium carbonate. Inanother example, the carbonate can be lithium carbonate. In yet anotherexample, the carbonate can be potassium carbonate. In yet anotherexample, the carbonate can be a mixture of at least two of sodiumcarbonate, lithium carbonate, and potassium carbonate.

In one aspect, as described herein, the system can comprise both H₂O anda carbonate as promoters.

In one aspect, the ratio of H₂O to supercritical CO₂ in thesupercritical CO₂ source is from 3.6*10⁻⁵% by weight to 1% by weight.For example, the ratio of H₂O to supercritical CO₂ in the supercriticalCO₂ source can be from 1.0*10⁻⁵% by weight to 1% by weight. In anotherexample, the ratio of H₂O to supercritical CO₂ in the supercritical CO₂source can be from 1.0*10⁻⁴% by weight to 1% by weight. In yet anotherexample, the ratio of H₂O to supercritical CO₂ in the supercritical CO₂source can be from 1.0*10⁻³% by weight to 1% by weight. In yet anotherexample, the ratio of H₂O to supercritical CO₂ in the supercritical CO₂source can be from 0.01% by weight to 1% by weight. In yet anotherexample, the ratio of H₂O to supercritical CO₂ in the supercritical CO₂source can be from 0.05% by weight to 1% by weight. In yet anotherexample, the ratio of H₂O to supercritical CO₂ in the supercritical CO₂source can be from 0.1% by weight to 1% by weight. In yet anotherexample, the ratio of H₂O to supercritical CO₂ in the supercritical CO₂source can be from 0.3% by weight to 1% by weight. In yet anotherexample, the ratio of H₂O to supercritical CO₂ in the supercritical CO₂source can be from 0.5% by weight to 1% by weight. In yet anotherexample, the ratio of H₂O to supercritical CO₂ in the supercritical CO₂source can be from 3.6*10⁻⁵% by weight to 0.5% by weight. In yet anotherexample, the ratio of H₂O to supercritical CO₂ in the supercritical CO₂source can be from 3.6*10⁻⁵% by weight to 0.3% by weight. In yet anotherexample, the ratio of H₂O to supercritical CO₂ in the supercritical CO₂source can be from 3.6*10⁻⁵% by weight to 0.1% by weight. In yet anotherexample, the ratio of H₂O to supercritical CO₂ in the supercritical CO₂source can be from 3.6*10⁻⁵% by weight to 0.05% by weight. In yetanother example, the ratio of H₂O to supercritical CO₂ in thesupercritical CO₂ source can be from 3.6*10⁻⁵% by weight to 0.01% byweight. In yet another example, the ratio of H₂O to supercritical CO₂ inthe supercritical CO₂ source can be from 3.6*10⁻⁵ by weight to 1.0*10⁻³%by weight. In yet another example, the ratio of H₂O to supercritical CO₂in the supercritical CO₂ source can be from 3.6*10⁻⁵% by weight to1.0*10⁻⁴% by weight.

In one aspect, the ratio of liquid carbonate to MgO in the reactor isfrom 1% by weight to 50% by weight. For example, the ratio of liquidcarbonate to MgO in the reactor is from 5% by weight to 50% by weight.In another example, the ratio of liquid carbonate to MgO in the reactoris from 10% by weight to 50% by weight. In yet another example, theratio of liquid carbonate to MgO in the reactor is from 20% by weight to50% by weight. In yet another example, the ratio of liquid carbonate toMgO in the reactor is from 30% by weight to 50% by weight. In yetanother example, the ratio of liquid carbonate to MgO in the reactor isfrom 1% by weight to 40% by weight. In yet another example, the ratio ofliquid carbonate to MgO in the reactor is from 1% by weight to 30% byweight. In yet another example, the ratio of liquid carbonate to MgO inthe reactor is from 1% by weight to 20% by weight. In yet anotherexample, the ratio of liquid carbonate to MgO in the reactor is from 1%by weight to 10% by weight. In yet another example, the ratio of liquidcarbonate to MgO in the reactor is from 1% by weight to 5% by weight.

In one aspect, solid particles can be present in the reactor. Such solidparticles can act as physical barriers to decrease agglomeration andsintering of the MgO active species, as compared to when no solidparticles are present. In one aspect, the solid particles can bealumina, silica, or carbon particles.

In one aspect, the CO₂ sorbent comprising MgO can be produced viapelletization. The pelletization can be done via a hydraulic press, forexample, without a binder. Such a pelletization technique can be used toincrease the energy density.

In one aspect, the CO₂ sorbent comprising MgO can be supported. Asupported CO₂ sorbent comprising MgO can be produced by impregnation.The support can be activated carbon, alpha-alumina, silica, vermiculite,or zirconia, or a mixture thereof. Such a support provides for stabilityand high surface area. See FIG. 9.

The supercritical CO₂ source can comprise any source known in the art.In some aspects, for example and without limitation the supercriticalCO₂ source can comprise a commercially available CO₂ provided in astorage tank, which can be in a supercritical state within thesupercritical CO₂ source, such as for example at about 74 atm at about31° C.

In one aspect, the supercritical CO₂ source can be located at leastpartially underground. In another aspect, the supercritical CO₂ sourcecan be located fully underground.

In one aspect, the CO₂ sorbent consists essentially of MgO. In anotheraspect, the CO₂ sorbent consists of MgO.

It is further understood that in some aspects, the CO₂ sorbent canoperate at temperatures greater than about 450° C., greater than about500° C., greater than about 550° C., greater than about 600° C., greaterthan about 650° C., greater than about 700° C., greater than about 750°C., greater than about 800° C., or greater than about 850° C. In yetother aspects, the CO₂ sorbents can operate in a temperature range fromabout 450° C. to about 900° C., including exemplary values of about 450°C., about 500° C., about 550° C., about 600° C., about 650° C., about700° C., about 750° C., about 800° C., about 850° C., and about 900° C.In yet other aspects, the CO₂ sorbents can operate in any temperaturerange between any two of the above stated values. For example, the CO₂sorbents can operate from about 450° C. to about 800° C., about 650° C.to about 850° C., or from about 700° C. to about 900° C.

The solid-phase MgO and MgCO₃ reactants are contained within thereactor, for example a single pressure reactor. In the discharged state,the reactor is filled with MgCO₃ at a reduced temperature, for example,from about 450° C. to about 650° C. To charge the reactor, CO₂ is heatedin part by the heat source to a temperature from about 600° C. to about900° C., for example, from about 650° C. to about 750° C., and a portionpasses through the reactor, which decomposes the MgCO₃ to MgO and CO₂.During energy discharge, the heat source can be bypassed allowing lowertemperature, for example, from about 450-650° C., CO₂ to enter thereactor. The CO₂ can be present in the reactor as a gas or in asupercritical stage. A portion of this CO₂ reacts with the MgO, andreleases heat, which increases the temperature of the remainder of thesupercritical CO₂, which can then be expanded through a turbine togenerate electricity.

It is understood that the pressure in the reactor is elevated. Forexample, the pressure in the reactor can be at least about 50 atm, atleast about 70 atm, at least about 100 atm, at least about 150 atm, atleast about 200 atm, at least about 250 atm, at least about 300 atm, orat least about 350 atm. In another example, the pressure in the reactorcan be from about 50 atm to about 400 atm, such for example from about50 atm to about 200 atm, from about 50 atm to about 100 atm, from about200 atm to about 400 atm, or from about 250 atm to about 350 atm. Thepressures and temperatures disclosed herein can be combined in asdesired. For example, during the discharge cycle the pressure can befrom about 50 atm to about 100 atm and have a temperature from about550° C. to about 650° C. In another example, during the charge cycle thepressure can be from about 250 atm to about 350 atm and have atemperature from about 650° C. to about 750° C.

In certain aspects, the CO₂ sorbent is a porous sorbent. In stillfurther aspects, the pores of the sorbent can have a diameter in therange from about 1 nm to about 200 nm, for example from about 5 nm toabout 200 nm, including exemplary values of about 3 nm, about 5 nm,about 8 nm, about 10 nm, about 15 nm, about 20 nm, about 25 nm, about 30nm, about 35 nm, about 40 nm, about 45 nm, about 50 nm, about 55 nm,about 60 nm, about 65 nm, about 70 nm, about 75 nm, about 80 nm, about85 nm, about 90 nm, about 95 nm, about 100 nm, about 120 nm, about 140nm, about 160 nm, and about 180 nm. In yet other aspects, the sorbentcomprises micropores having a diameter in the range from about 1 nm toabout 10 nm, including exemplary values of about 2 nm, about 3 nm, about4 nm, about 5 nm, about 6 nm, about 7 nm, about 8 nm, and about 9 nm. Instill further aspects, the micropores can have a diameter between anytwo of the above stated values. In yet other aspects, the sorbent cancomprise mesopores having a diameter in the range from about 10 nm toabout 100 nm, including exemplary values of about 20 nm, about 30 nm,about 40 nm, about 50 nm, about 60 nm, about 70 nm, about 80 nm, andabout 90 nm. In still further aspects, the sorbent can comprisemacropores with a diameter greater than about 100 nm. It is understoodthat in some aspects, the greater pore diameter can result in adecreased total pore surface area.

In certain aspects, the CO₂ sorbent described herein have a surface areain the range from about 1 to about 1,000 m²/g, including exemplaryvalues ranging from of about 5 to 50 m²/g.

It is further understood that the pore diameter of the sorbent can beaffected by a temperature. In some aspects, at the higher reactiontemperatures the sorbent can undergo sintering and form agglomerateshaving a higher pore size.

In certain aspects, the CO₂ sorbent can be heat treated in anon-reacting gas such as nitrogen, air, or helium prior to the use in asystem at a temperature from about 600° C. to about 1,000° C., includingexemplary values of about 650° C., about 700° C., about 750° C., about800° C., about 850° C., about 900° C., and about 950° C., therebyproducing a heat treated sorbent. In some aspects, the heat treatment isperformed for a period of time from about 10 minutes to about 60minutes, including exemplary values of about 20 minutes, about 30minutes, about 40 minutes, and about 50 minutes. In yet other aspects,the sorbent is heat treated in a steam. In yet other aspects, followingthis heat treatment, the sorbent can be further heat treated in a gascontaining about 2 to about 30 volume % CO₂ from about 600° C. to about800° C., including exemplary values of about 650° C., about 700° C., andabout 750° C. In yet other aspects, the heat treatment can be done for atime period of about 4 to about 20 hours, including exemplary values ofabout 6, about 8, and about 12 hours. In certain aspects, the heattreatment of the sorbent prior to the use in a system improves thesorbent durability and stability. In certain aspects, the heat treatmentof the sorbent can improve a reaction rate (e.g. to increase the amountof CO₂ that can be reacted with the sorbent in a given time). In certainexemplary aspects, the absorption of CO₂ during the first reaction cyclelasting for about an can be increased from about 10 wt % to about 36 wt%, including exemplary values of about 15 wt. %, about 20 wt %, about 25wt. %, about 30 wt. %, and about 35 wt. %, comparatively to asubstantially identical sorbent that was not heat treated prior to usein a system. Without wishing to be bound by any theory, it ishypothesized that the heat treating of the sorbent prior to the use in asystem can activate the sorbent by structuring the surface morphologyand increasing access of the CO₂ to the pore structure.

In yet other aspects, the MgCO₃ can undergo a regeneration process at atemperature from about 450° C. to about 950° C., for example from about550° C. to about 800° C., about 600° C. to about 800° C. includingexemplary values of about 550° C., of about 600° C., about 700° C.,about 750° C., about 800° C., about 850° C., and about 900° C., which isachieved with heated CO₂. This regeneration of MgCO₃ can be performedfor a time period of about 30 minutes to about 12 hours, includingexemplary values of about 1 hour, about 1.5 hour, about 2 hours, about2.5 hours, about 3 hours, about 3.5 hours, about 4 hours, about 4.5hours, about 5 hours, about 5.5 hours, about 6 hours, about 6.5 hours,about 7 hours, about 7.5 hours, about 8 hours, about 8.5 hours, about 9hours, about 9.5 hours, about 10 hours, about 10.5 hours, about 11hours, and about 11.5 hours, in the presence of CO₂. In some aspects,the regeneration process can convert the magnesium carbonate (MgCO₃)back to magnesium oxide (MgO) to make it ready for the next cycle.Without wishing to be bound by any theory it is hypothesized that theregeneration process can rejuvenate the sorbent to its activated statewithout causing a loss in sorbent durability so that cycles can berepeated multiple times. It is further understood that the regenerationprocess can be conducted in the presence of CO₂.

In certain aspects, the CO₂ sorbent described herein can withstand fromabout 100 to about 30,000 reaction cycles without any substantialdegradation, including exemplary values of about 200, about 500, about1,000, about 1,500, about 2,000, about 2,500, about 3,000, about 3,500,about 4,000, about 4,500, about 5,500, about 6,000, about 6,500, about7,000, about 7,500, about 8,000, about 8,500, about 9,000, about 9,500,about 10,000, about 10,500, about 11,000, about 11,500, about 12,000,about 12,500, about 13,000, about 13,500, about 14,000, about 14,500,about 15,500, about 16,000, about 16,500, about 17,000, about 17,500,about 18,000, about 18,500, about 19,000, about 19,500, about 20,000,about 20,500, about 21,000, about 21,500, about 22,000, about 22,500,about 23,000, about 23,500, about 24,000, about 24,500, about 25,500,about 26,000, about 26,500, about 27,000, about 27,500, about 28,000,about 28,500, about 29,000, and about 29,500. In yet other aspects, theCO₂ sorbent described herein can withstand any number of cycles inbetween any cited above values without any substantial degradation. Insome aspects, the CO₂ sorbent can withstand from about 1,000 to about20,000 cycles or from about 5,000 to about 30,000 cycles without anysubstantial degradation.

In certain aspects, the absence of the substantial degradation of theCO₂ sorbent can be determined by an amount of the CO₂ that can reactwith the CO₂ sorbent in each consequent reaction cycle conducted afterthe first cycle as compared to an amount of the CO₂ reacted with the CO₂sorbent in the first cycle. In some aspects, the amount of the CO₂reacted with the CO₂ sorbent in each consequent reaction cycle conductedafter the first cycle is at least about 50% of the CO₂ reacted with theCO₂ sorbent in the first cycle, at least about 55%, at least about 60%,at least about 65%, at least about 70%, at least about 75%, at leastabout 80%, at least about 85%, at least about 90%, at least about 95%,at least about 99%, or at least about 99.8% of CO₂ reacted with the CO₂sorbent in the first cycle.

In some aspects, the reactor described herein can comprise a heatexchange reactor or an adiabatic reactor. An example of a design of areactor disclosed herein is shown in FIG. 2. In certain aspects, thereactor is a heat exchange reactor. In other aspects, the reactor is anadiabatic reactor. It is understood that any heat exchange and adiabaticreactors known in the art can be utilized. It is further understood thatthe reactor can have any shape known in the art. In some aspects, thereactor is a tube. In other aspects, the reactor is a shell. In yetother aspects, the reactor is a shell and tube reactor or afluidized-bed reactor. It is further understood that the dimensions ofthe reactor can be easily determined by one of ordinary skill in the artdepending on the desired outcome. In some aspects, it is understood thatthe size of the reactor can be fitted to house a sufficient amount ofCO₂ sorbent effective to react with a desired amount of CO₂. Forexample, an excess amount of CO₂ sorbent can be present in the reactorrelative to the amount of CO₂ that is introduced into the reactor,thereby maximizing the efficiency of the CO₂ that is introduced into thereactor

In some aspects, the system can further comprise a heat sourceconfigured to be in fluid communication with the supercritical CO₂source and the reactor.

In certain aspects, the heat source present in the system can comprise asolar thermal energy source; for example. In the exemplary aspects,wherein the heat source comprises a solar thermal energy source, thesolar energy can be concentrated and directed using mirrors for directheating of the supercritical CO₂. In yet other aspects, the heat sourcepresent in the system can increase the temperature of the supercriticalCO₂ to a temperature of at least about 500° C., at least about 550° C.,at least about 600° C., at least about 650° C., at least about 700° C.,at least about 750° C., at least about 800° C., at least about 850° C.,at least about 900° C., or at least about 950° C. For example, the heatsource present in the system can increase the temperature of thesupercritical CO₂ to a temperature of from about 500° C. to about 950°C., for example from about 600° C. to about 800° C. In yet otheraspects, the heat source can comprise any heat source known in the art,for example and without limitation, a gas fire plant, a nuclear reactor,and the like. The supercritical CO₂ that has been heated by the heatsource is then introduced into the reactor.

In another aspect, the system can further comprise a pump configured topump supercritical CO₂ from the supercritical CO₂ source towards theheat source and/or reactor. The supercritical CO₂ that exits thesupercritical CO₂ source can be compressed in a compressor and is in adense form. Accordingly, the system can further comprise a compressorconfigured to be in fluid communication with the supercritical CO₂source and the heat source and/or reactor. It is suitable for a pump totransfer this dense supercritical CO₂ towards the heat source and/orreactor.

In another aspect, the system can further comprise one or more heatexchangers configured to be in fluid communication with thesupercritical CO₂ source, the reactor, and the heat source. Thesupercritical CO₂ that is pumped towards the heat source and/or reactorpasses through the one or more heat exchangers which has a highertemperature than the supercritical CO₂, thereby increasing thetemperature of the supercritical CO₂. The temperature of the than thesupercritical CO₂ can be increased by the one or more heat exchangers toa temperature from about 450° C. to about 700° C. During discharge thissupercritical CO₂ having a temperature from about 450° C. to about 650°C. is used directly to react CO₂ with MgO to produce MgCO₃ and heat.Said differently, this supercritical CO₂ having a temperature from about450° C. to about 650° C. is used in the reactor without being furtherheated by the heat source. During the charge supercritical CO₂ having atemperature from about 450° C. to about 650° C. is further heated by theheat source before use in the reactor to drive the endothermicdegradation of MgCO₃ to MgO and CO₂.

In another aspect, the system can further comprise a turbine configuredto be in fluid communication with an outlet of the reactor. During thedischarge cycle the CO₂, which has been heated by the exothermicreaction in the reactor, exits the reactor and is expanded in theturbine to produce electricity.

In another aspect, the system can further comprise a cooling unitconfigured to be in fluid communication with an outlet of the reactorand the supercritical CO₂ source. The cooling unit is used to cool CO₂so it can be stored in the supercritical CO₂ source. The heat absorbedby the cooling unit can later be used to heat the supercritical CO₂present in the supercritical CO₂ source to be used in the subsequentcycle.

In another aspect, the system further comprises a cooling unitconfigured to be in fluid communication with an outlet of the reactor,the one or more heat exchangers, and the supercritical CO₂ source. Insuch an aspect, the cooling unit is configured to receive CO₂ after theCO₂ has passed through the one or more heat exchangers from the outletof the reactor.

In another aspect, the system can further comprise a sensible heatstorage unit configured to be in fluid communication with thesupercritical CO₂ source and the heat source and/or reactor. Thesensible heat storage unit is configured to receive CO₂ from a chargingcycle, for example, as shown in FIG. 1. The sensible heat storage unitreduces the temperature of the CO₂ before the CO₂ is transferred to thesupercritical CO₂ source for storage. The heat absorbed in the sensibleheat storage unit can be used heat the supercritical CO₂ in thesupercritical CO₂ source before use in the next cycle.

In another aspect, the system disclosed herein is a closed loop system.

In another aspect, the system is an industrial sized system.

FIG. 1 shows an exemplary thermochemical storage energy system 100. Thisexemplary thermochemical storage energy system 100 is comprised of areactor 102, a supercritical CO₂ source 104, heat source 106, one ormore heat exchangers 108, a turbine 110, a cooling unit 112, a sensibleheat storage unit 114, a low temperature compressor 116, and a hightemperature compressor 118. The reactor 102, for example a singlepressure reactor, contains a fixed bed of the CO₂ sorbent comprisingMgO. Supercritical CO₂ is stored in the supercritical CO₂ source 104 atabout 74 atm at about 31° C. The supercritical CO₂ is heated to about100° C. using heat absorbed in the cooling unit 112 or the sensible heatstorage unit 114 or a combination thereof. The heated supercritical CO₂is compressed in a low temperature compressor 116 and further heated inthe one or more heat exchangers 108. To discharged the reactor 102, thesupercritical CO₂, which has a from about 450° C. to about 650° C., forexample from about 550° C. to about 600° C., bypasses the heat source106 and enters the reactor, for example at a pressure around 10 atm. TheCO₂ can be in the form of a gas or at a supercritical state in thereactor 102. A portion of the CO₂ reacts with the MgO present in the CO₂sorbent to form MgCO₃ and release heat, i.e. this reaction isexothermic. The heat increases the temperature of the unreacted CO₂,which is next expanded in the turbine 110, which generates electricity.The expanded unreacted CO₂ is cooled in the one or more heat exchangers108 and the cooling unit 112 before entering the supercritical CO₂source 104 to be stored at about 74 atm at about 31° C.

To charge the reactor 102, supercritical CO₂ is again heated to about100° C. using heat absorbed in the cooling unit 112 or the sensible heatstorage unit 114 or a combination thereof. The heated supercritical CO₂is again compressed in a low temperature compressor 116 and furtherheated in the one or more heat exchangers 108. This time, thesupercritical CO₂ is even further heated in the heat source 106 to atemperature from about 600° C. to about 800° C., for example, from about650° C. to about 750° C. This heated CO₂ (supercritical or gas orcombination thereof) enters the reactor 102 to decompose the MgCO₃ toMgO and CO₂. This CO₂ is separated from the sorbent and transferred ofthe reactor together with the heated CO₂ and is transferred to asensible heat storage unit 114 to be cooled. The CO₂ is then furthercooled in the in the one or more heat exchangers 108 and the coolingunit 112 before entering the supercritical CO₂ source 104 to be storedat about 74 atm at about 31° C.

As shown in FIG. 2 an isothermic cycle proceeds counter-clockwise infour stages 1) decarbonation of MgCO₃ to charge the system with latentheat 2) raising the temperature of the MgO-based sorbent with sensibleheat 3) discharging the system of latent heat by carbonation at 675° C.4) discharging the system of sensible heat.

FIG. 3 shows an isothermal cycle of a system and method disclosedherein. FIG. 3 shows that the system and method disclosed here can beoperated with low temperature differences, which is beneficial to theefficiency of the system.

The exergy of a system disclosed herein with all irreversibles orreactor irreversibles only is shown in FIG. 5.

3. Methods

Disclosed herein is a method for storing energy. In some aspectsdisclosed herein is a method of storing energy comprising the steps of:a) in a reactor, in the presence of H₂O and/or a carbonate, heatingMgCO₃ with supercritical CO₂ having a temperature of at least 450° C.,thereby promoting an endothermic chemical reaction to produce CO₂ andMgO; and b) separating the CO₂ from the MgO.

Also disclosed herein is a method of storing energy comprisingcontacting MgCO₃ with supercritical CO₂ having a temperature of at least450° C. in the presence of H₂O and/or a carbonate in the systemdisclosed herein, to thereby store energy.

In one aspect, the heating the MgCO₃ with supercritical CO₂ is in thepresence of H₂O.

In one aspect, the method can further comprise a step c) combiningsupercritical CO₂ having a temperature of less than about 700° C. withthe MgO in the reactor, thereby promoting an exothermic chemicalreaction to produce heat and MgCO₃. In one aspect, the produced heat canincrease the temperature of unreacted supercritical CO₂ to produceheated unreacted supercritical CO₂, and wherein the heated unreactedsupercritical CO₂ is expanded in a turbine to generate electricity. Inyet another aspect, the expanded heated unreacted supercritical CO₂ canbe cooled via one or more heat exchangers and a cooling unit beforebeing stored in a supercritical CO₂ source.

Energy is stored as a potential future chemical reaction between CO₂ andthe MgO in the CO₂ sorbent. As described elsewhere herein, the chemicalreaction between CO₂ and the MgO in the CO₂ sorbent is exothermic and,energy in the form of heat is released.

In one aspect, the method can further comprise transporting at least aportion of the separated CO₂ to a supercritical CO₂ source via one ormore heat exchangers and a cooling unit. The MgO in the CO₂ sorbent is,once separated and not in the presence of CO₂, available to berecombined with CO₂ to form MgCO₃.

It is understood that the reactor can comprise any reactor describedherein. For example and without limitation it can comprise a heatexchange reactor, or an adiabatic reactor. In certain aspects, the CO₂sorbent can comprise any CO₂ sorbent described herein.

In certain aspects, the method comprising steps (a) and (b) or themethod comprising steps (a) through (c) of the disclosed method can berepeated for at least about 100 times, for at least about 200 times, atleast about 500 times, at least about 1,000 times, at least about 1,500times, at least about 2,000 times, at least about 2,500 times, at leastabout 3,000 times, at least about 3,500 times, at least about 4,000times, at least about 4,500 times, at least about 5,500 times, at leastabout 6,000 times, at least about 6,500 times, at least about 7,000times, at least about 7,500 times, at least about 8,000 times, at leastabout 8,500 times, at least about 9,000 times, at least about 9,500times, at least about 10,000 times, at least about 10,500 times, atleast about 11,000 times, at least about 11,500 times, at least about12,000 times, at least about 12,500 times, at least about 13,000 times,at least about 13,500 times, at least about 14,000 times, at least about14,500 times, at least about 15,500 times, at least about 16,000 times,at least about 16,500 times, at least about 17,000 times, at least about17,500 times, at least about 18,000 times, at least about 18,500 times,at least about 19,000 times, at least about 19,500 times, at least about20,000 times, at least about 20,500 times, at least about 21,000 times,at least about 21,500 times, at least about 22,000 times, at least about22,500 times, at least about 23,000 times, at least about 23,500 times,at least about 24,000 times, at least about 24,500 times, at least about25,500 times, at least about 26,000 times, at least about 26,500 times,at least about 27,000 times, at least about 27,500 times, at least about28,000 times, at least about 28,500 times, at least about 29,000 times,at least about 29,500 times, or at least about 30,000 times.

In yet other aspects, the method comprising steps (a) through (c) can berepeated at least 1,000 time, wherein the amount of CO₂ can be reactedwith the CO₂ sorbent in step c) throughout the method is at least 50% ofthe amount of CO₂ that could be reacted with the CO₂ sorbent prior toperforming the method. In some aspects, the amount of CO₂ that could bereacted with the CO₂ sorbent is at least about 50%, at least about 55%,at least about 60%, at least about 65%, at least about 70%, at leastabout 75%, at least about 80%, at least about 85%, at least about 90%,at least about 95%, or at least about 99% of the amount of CO₂ thatcould be reacted with the CO₂ sorbent prior to performing the method.

In certain aspects, the steps (a) through (c) can be repeated from 1,000to 20,000 times, wherein the amount of CO₂ that can be reacted with theCO₂ sorbent in step c) throughout the method is at least 50% of theamount of CO₂ that could be reacted with the CO₂ sorbent prior toperforming the method. In some aspects, the amount of CO₂ that could bereacted with the CO₂ sorbent is at least about 50%, at least about 55%,at least about 60%, at least about 65%, at least about 70%, at leastabout 75%, at least about 80%, at least about 85%, at least about 90%,at least about 95%, or at least about 99% of the amount of CO₂ thatcould be reacted with the CO₂ sorbent prior to performing the method.

ASPECTS

In view of the described systems and methods and variations thereof,herein below are described certain more particularly described aspectsof the inventions. These particularly recited aspects should not howeverbe interpreted to have any limiting effect on any different claimscontaining different or more general teachings described herein, or thatthe “particular” aspects are somehow limited in some way other than theinherent meanings of the language and formulas literally used therein.

Aspect 1: A system for storing energy comprising: a) a reactorcomprising a CO₂ sorbent comprising MgO; and b) a supercritical CO₂source comprising supercritical CO₂ and H₂O, wherein the supercriticalCO₂ source is in fluid communication with the reactor and the CO₂sorbent comprising MgO to allow flow of the supercritical CO₂ and H₂Obetween the supercritical CO₂ source and the reactor, thereby allowingcontact of CO₂ with the CO₂ sorbent comprising MgO.

Aspect 2: A system for storing energy comprising: a) a reactorcomprising a CO₂ sorbent comprising MgO and a liquid carbonate promoter;and b) a supercritical CO₂ source comprising supercritical CO₂, whereinthe supercritical CO₂ source is in fluid communication with the reactorand the CO₂ sorbent comprising MgO to allow flow of the supercriticalCO₂ between the supercritical CO₂ source and the reactor, therebyallowing contact of CO₂ with the CO₂ sorbent comprising MgO.

Aspect 3: The system of aspect 1, wherein the ratio of H₂O tosupercritical CO₂ in the supercritical CO₂ source is from 3.6*10⁻⁵% byweight to 1% by weight.

Aspect 4: The system of aspect 2, wherein the ratio of liquid carbonateto MgO in the reactor is from 1% by weight to 50% by weight.

Aspect 5: The system of aspects 2 or 4, wherein the carbonate isselected from the group consisting of sodium carbonate, lithiumcarbonate, and potassium carbonate, or a mixture thereof.

Aspect 6: The system of any one of aspects 1-5, wherein the systemfurther comprises a heat source configured to be in fluid communicationwith the supercritical CO₂ source and the reactor.

Aspect 7: The system of aspect 6, wherein the system further comprises apump configured to pump supercritical CO₂ from the supercritical CO₂source towards the heat source and/or reactor.

Aspect 8: The system of aspect 7, wherein the system further comprisesone or more heat exchangers configured to be in fluid communication withthe supercritical CO₂ source, the reactor, and the heat source.

Aspect 9: The system of any one of aspects 1-8, wherein the systemfurther comprises a turbine configured to be in fluid communication withan outlet of the reactor.

Aspect 10: The system of any one of aspects 6-9, wherein the systemfurther comprises a compressor configured to be in fluid communicationwith the supercritical CO₂ source and the heat source and/or reactor.

Aspect 11: The system of any one of aspects 1-10, wherein the systemfurther comprises a cooling unit configured to be in fluid communicationwith an outlet of the reactor and the supercritical CO₂ source.

Aspect 12: The system of any one of aspects 8-11, wherein the systemfurther comprises a cooling unit configured to be in fluid communicationwith an outlet of the reactor, the one or more heat exchangers, and thesupercritical CO₂ source.

Aspect 13: The system of any one of aspects 6-12, wherein the systemfurther comprises a sensible heat storage unit configured to be in fluidcommunication with the supercritical CO₂ source and the heat sourceand/or reactor.

Aspect 14: The system of any one of aspects 1-13, wherein the reactor isa heat exchange reactor.

Aspect 15: The system of any one of aspects 6-14, wherein the heatsource is a solar thermal energy source.

Aspect 16: The system of any one of aspects 1-14, wherein the system isa closed loop system.

Aspect 17: A method of storing energy comprising the steps of: a) in areactor, in the presence of H₂O and/or a carbonate, heating MgCO₃ withsupercritical CO₂ having a temperature of at least 450° C., therebypromoting an endothermic chemical reaction to produce CO₂ and MgO; andb) separating the CO₂ from the MgO.

Aspect 18: The method of aspect 17, wherein heating the MgCO₃ withsupercritical CO₂ is in the presence of H₂O.

Aspect 19: The method of any one of aspects 17 or 18, wherein heatingthe MgCO₃ with supercritical CO₂ is in the presence of a carbonate.

Aspect 20: The method of any one of aspects 17-19, wherein the methodfurther comprises transporting at least a portion of the separated CO₂to a supercritical CO₂ source via one or more heat exchangers and acooling unit.

Aspect 21: The method of any one of aspects 17-20, wherein the methodfurther comprises step c) in the presence of H₂O and/or a carbonate,combining supercritical CO₂ having a temperature of less than about 700°C. with the MgO in the reactor, thereby promoting an exothermic chemicalreaction to produce heat and MgCO₃.

Aspect 22: The method of aspect 21, wherein the heat increases thetemperature of unreacted supercritical CO₂ to produce heated unreactedsupercritical CO₂, and wherein the heated unreacted supercritical CO₂ isexpanded in a turbine to generate electricity.

Aspect 23: The method of aspect 22, wherein the expanded heatedunreacted supercritical CO₂ is cooled via one or more heat exchangersand a cooling unit before being stored in a supercritical CO₂ source.

Aspect 24: The method of aspect 20, wherein the method furthercomprises, prior to step a), increasing the temperature of thesupercritical CO₂ in a supercritical CO₂ source by transferring heatfrom the cooling unit and/or a latent heat storage unit to thesupercritical CO₂ source.

Aspect 25: The method of any one of aspects 21-24, wherein steps a)-c)are repeated at least 1,000 times, wherein the amount of CO₂ that can bereacted with the MgO in step c) throughout the method is at least 50% ofthe amount of CO₂ that could be reacted with the MgO prior to performingthe method.

Aspect 26: The method of any one of aspects 21-24, wherein steps a)-c)are repeated from 1,000 to 20,000 times, wherein the amount of CO₂ thatcan be reacted with the MgO in step c) throughout the method is at least50% of the amount of CO₂ that could be reacted with the MgO prior toperforming the method.

Aspect 27: A method of storing energy comprising contacting MgCO₃ withsupercritical CO₂ having a temperature of at least 450° C. in thepresence of H₂O and/or a carbonate in the system of aspect 1, to therebystore energy.

EXAMPLES

The following examples are put forth so as to provide those of ordinaryskill in the art with a complete disclosure and description of how thecompounds, compositions, articles, devices and/or methods claimed hereinare made and evaluated, and are intended to be purely exemplary and arenot intended to limit the disclosure. Efforts have been made to ensureaccuracy with respect to numbers (e.g., amounts, temperature, etc.), butsome errors and deviations should be accounted for. Unless indicatedotherwise, parts are parts by weight, temperature is in ° C. or is atambient temperature, and pressure is at or near atmospheric.

Example 1

A reactor, see FIG. 10, was used to validate the CO₂ mass balancetechnique and the MgO/MgCO₃ equilibrium curve of MgO. A Na/K/Licarbonate salt blend (at 30 wt %) was also added to the reactor. FIG. 11shows the run chart for four cycles at 4 hr cycle times at anequilibrium temperature around 650° C. Such a condition is commerciallyrelevant. The temperature was measured from outside the vessel insteadof directly at the sorbent, therefore the ΔT to drive the process isreported larger than what the actual ΔT should be. The shape of thepressure curve suggests a transition from 1st (island growth) to 2nd(diffusion limited) reaction regimes. Therefore, it can be concludedthat the carbonate salt exhibits desired kinetics.

A MgO sorbent was synthesized. No promoter was added to the MgO sorben.As a screening before testing the sorbent was held in a furnace at 650°C. in air (decarbonated state) and the surface area was measured at 16,32, and 80 hr to be 49.86, 49.76 and 50.36 m²/g, respectively. Thesurface area of this MgO sorbent is stable at 10 x the magnitude of acomparable CaO sorbent, which has recently demonstrated no long-termdegradation under 600 accelerated cycles in the thermogravimetricanalysis (TGA). The stability is not unexpected because the meltingtemperature of MgO is 239° C. higher than CaO, which results in a lowerhomologous temperature T_(H)=T/T_(melt) and less mass diffusion andsintering. The tapped density was measured to be 834 kg/m³. The systemwas commissioned and the experiment ran for 2 days. Leaks at the hightemperature seals limited the time between recharges to ˜8 hrs. Thesystem design, instrumentation, operation and sensitivity of theanalysis method were all proven successful. The sorbent performance canbe summarized by a single observed decarbonation half cycle beginning at126 atm and 615.8° C. The sorbent released 4.31 g of CO₂ over 29.5 minwhich equates to a CO₂ weight gain of 26.9%. This is a lower boundestimate if the simultaneous system leak rate was accounted for, thismass of CO₂ released would be higher.

The following conclusions can be drawn: 1) the equilibrium curvepredicted by thermochemical modeling is verified (FIG. 6), 2) thereaction kinetics are fast enough that a promoter may not be necessary3) the overall energy capacity equates to 547 MJ/m³ which is within TEAtargets (FIG. 7), 4) the low-cost manufacturing method is suitable forthis application. Although the long-term durability of the sorbent hasnot been characterized, there are still many parameters to optimize forincreased performance, including the addition of promoters.

It was also demonstrated a stable weight gain of 25% for the MgO-basedsorbent at 350° C. (FIG. 8).

What is claimed is:
 1. A system for storing energy comprising: a) areactor comprising a CO₂ sorbent comprising MgO; and b) a supercriticalCO₂ source comprising supercritical CO₂ and H₂O, wherein thesupercritical CO₂ source is in fluid communication with the reactor andthe CO₂ sorbent comprising MgO to allow flow of the supercritical CO₂and H₂O between the supercritical CO₂ source and the reactor, therebyallowing contact of CO₂ with the CO₂ sorbent comprising MgO.
 2. A systemfor storing energy comprising: a) a reactor comprising a CO₂ sorbentcomprising MgO and a liquid carbonate promoter; and b) a supercriticalCO₂ source comprising supercritical CO₂, wherein the supercritical CO₂source is in fluid communication with the reactor and the CO₂ sorbentcomprising MgO to allow flow of the supercritical CO₂ between thesupercritical CO₂ source and the reactor, thereby allowing contact ofCO₂ with the CO₂ sorbent comprising MgO.
 3. The system of claim 1,wherein the ratio of H₂O to supercritical CO₂ in the supercritical CO₂source is from 3.6*10⁻⁵% by weight to 1% by weight.
 4. The system ofclaim 2, wherein the ratio of liquid carbonate to MgO in the reactor isfrom 1% by weight to 50% by weight.
 5. The system of claim 2, whereinthe carbonate is selected from the group consisting of sodium carbonate,lithium carbonate, and potassium carbonate, or a mixture thereof.
 6. Thesystem of claim 1, wherein the system further comprises a heat sourceconfigured to be in fluid communication with the supercritical CO₂source and the reactor.
 7. The system of claim 6, wherein the systemfurther comprises a pump configured to pump supercritical CO₂ from thesupercritical CO₂ source towards the heat source and/or reactor.
 8. Thesystem of claim 7, wherein the system further comprises one or more heatexchangers configured to be in fluid communication with thesupercritical CO₂ source, the reactor, and the heat source.
 9. Thesystem of claim 1, wherein the system further comprises a turbineconfigured to be in fluid communication with an outlet of the reactor.10. The system of claim 1, wherein the system further comprises acooling unit configured to be in fluid communication with an outlet ofthe reactor and the supercritical CO₂ source.
 11. The system of claim 8,wherein the system further comprises a cooling unit configured to be influid communication with an outlet of the reactor, the one or more heatexchangers, and the supercritical CO₂ source.
 12. The system of claim 6,wherein the system further comprises a sensible heat storage unitconfigured to be in fluid communication with the supercritical CO₂source and the heat source and/or reactor.
 13. The system of claim 1,wherein the reactor is a heat exchange reactor.
 14. The system of claim6, wherein the heat source is a solar thermal energy source.
 15. Thesystem of claim 1, wherein the system is a closed loop system.
 16. Amethod of storing energy comprising the steps of: a) in a reactor, inthe presence of H₂O and/or a carbonate, heating MgCO₃ with supercriticalCO₂ having a temperature of at least 450° C., thereby promoting anendothermic chemical reaction to produce CO₂ and MgO; and b) separatingthe CO₂ from the MgO.
 17. The method of claim 16, wherein the methodfurther comprises transporting at least a portion of the separated CO₂to a supercritical CO₂ source via one or more heat exchangers and acooling unit.
 18. The method of claim 16, wherein the method furthercomprises step c) in the presence of H₂O and/or a carbonate, combiningsupercritical CO₂ having a temperature of less than about 700° C. withthe MgO in the reactor, thereby promoting an exothermic chemicalreaction to produce heat and MgCO₃.
 19. The method of claim 18, whereinsteps a)-c) are repeated from 1,000 to 20,000 times, wherein the amountof CO₂ that can be reacted with the MgO in step c) throughout the methodis at least 50% of the amount of CO₂ that could be reacted with the MgOprior to performing the method.
 20. A method of storing energycomprising contacting MgCO₃ with supercritical CO₂ having a temperatureof at least 450° C. in the presence of H₂O and/or a carbonate in thesystem of claim 1, to thereby store energy.